# Converting constructor

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< cpp‎ | language

A constructor that is not declared with the specifier explicit and which can be called with a single parameter(until C++11) is called a converting constructor.

Unlike explicit constructors, which are only considered during direct initialization (which includes explicit conversions such as static_cast), converting constructors are also considered during copy initialization, as part of user-defined conversion sequence.

It is said that a converting constructor specifies an implicit conversion from the types of its arguments (if any) to the type of its class. Note that non-explicit user-defined conversion function also specifies an implicit conversion.

Implicitly-declared and user-defined non-explicit copy constructors and move constructors are converting constructors.

### Example

Run this code

    struct A
    {
        A() { }         // converting constructor (since C++11)
        A(int) { }      // converting constructor
        A(int, int) { } // converting constructor (since C++11)
    };

    struct B
    {
        explicit B() { }
        explicit B(int) { }
        explicit B(int, int) { }
    };

    int main()
    {
        A a1 = 1;      // OK: copy-initialization selects A::A(int)
        A a2(2);       // OK: direct-initialization selects A::A(int)
        A a3{4, 5};    // OK: direct-list-initialization selects A::A(int, int)
        A a4 = {4, 5}; // OK: copy-list-initialization selects A::A(int, int)
        A a5 = (A)1;   // OK: explicit cast performs static_cast, direct-initialization

    //  B b1 = 1;      // error: copy-initialization does not consider B::B(int)
        B b2(2);       // OK: direct-initialization selects B::B(int)
        B b3{4, 5};    // OK: direct-list-initialization selects B::B(int, int)
    //  B b4 = {4, 5}; // error: copy-list-initialization selected an explicit constructor
                       //        B::B(int, int)
        B b5 = (B)1;   // OK: explicit cast performs static_cast, direct-initialization
        B b6;          // OK, default-initialization
        B b7{};        // OK, direct-list-initialization
    //  B b8 = {};     // error: copy-list-initialization selected an explicit constructor
                       //        B::B()

        [](...){}(a1, a4, a4, a5, b5); // may suppress "unused variable" warnings
    }

### See also

- copy assignment
- copy constructor
- copy elision
- default constructor
- destructor
- `explicit`
- initialization
  - aggregate initialization
  - constant initialization
  - copy initialization
  - default initialization
  - direct initialization
  - initializer list
  - list initialization
  - reference initialization
  - value initialization
  - zero initialization
- move assignment
- move constructor
- new

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